Ideas for a bridge replacemnt?

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gregh
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Ideas for a bridge replacemnt?

Post by gregh » Mon Aug 03, 2020 10:08 am

Way back in 1994 when I started building my railway, the 2nd bridge I built was this timber truss. I was really proud of it as it is really all held together by the tension rods just like the real thing.
truss side.JPG
truss side.JPG (271.83 KiB) Viewed 5654 times

It’s about 1.2m long, made from tomato stakes. I relocated it slightly in 2003 due to a track realignment, so that it was very close to an overhanging gum tree trunk. Over the years, the tree has grown and leant over so it is pushing on the bridge. I have been cutting into the thick bark for a few years to give clearance, but now I am into the hard wood.
treeonbridge.JPG
treeonbridge.JPG (165.07 KiB) Viewed 5654 times

So it is time for drastic action. My first idea was to just invert the truss so the track ran across the top, but some measurements revealed not enough clearance under the bridge at one end. It looks like a new bridge is needed. What a shame – the timber is in perfect condition despite being 26 years young.
I would love to do a masonry or steel arch under the track but it’s a lot of work because the two support ends are different heights.


So I think the simplest design would be a steel girder like this (in styrene) …
girder1.jpg
girder1.jpg (9.04 KiB) Viewed 5654 times
but I don’t like the absence of ‘safety rails to catch derailed trains! It's a 1m drop to rock for a derailed train!

So something like this through plate girder would be better…
A quick look through this website, let me determine some of the dimensions of the girder.
https://theconstructor.org/structural-e ... dges/2066/
Depth D= L/10
Being 1.2m long would mean a girder depth D of around 120mm.
Flange width D/3 to D/4 (30-40mm). Web width D/125 so only 1mm !!
So it would look something like this:
girder.jpg
girder.jpg (91.9 KiB) Viewed 5654 times
But a lot of work cutting and ‘riveting’ all those angle irons.


I do have a ‘spare’ steel arch bridge that is 6’ long and if I cut it down to 4’ it is only about 5” high at the centre.
arch.jpg
arch.jpg (177.03 KiB) Viewed 5654 times
But it really means completely dismantling the bridge and rebuilding. Is it worth the work??
Maybe just put a piece of timber under the track and forget it??
Greg from downunder.
The Sandstone & Termite's website: https://members.optusnet.com.au/satr/satr.htm

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Andrew
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Re: Ideas for a bridge replacemnt?

Post by Andrew » Mon Aug 03, 2020 12:36 pm

Hello!

I reckon either option would look good and definitely be worth the effort, but I wonder whether you could modify the existing bridge by simply cutting out the bits where it contacts with the tree? Yes, it would look a little odd (like the tree had somehow grown through the bridge) if you stopped to really look, but how often does anyone do that? Most of the time you'd glance over, your eye would see the bridge and the tree, and your mind would adjust the picture to make it all work... Of course it may not be possible with the design of the bridge - I'm more of an artist than an engineer!

Good luck with it, whatever you decide,

Andrew.

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Re: Ideas for a bridge replacemnt?

Post by invicta280 » Mon Aug 03, 2020 10:36 pm

You're up a gum tree here.... sorry. That tree is at a heck of an angle by the look of it. Does it sway much in the wind ? That could affect the alignment. Is there no way you could keep the existing bridge? It is clearly a goodun and one that you are fond of. Can the approach to the bridge be realigned some way so that bridge is further from the tree trunk?

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Re: Ideas for a bridge replacemnt?

Post by ge_rik » Tue Aug 04, 2020 9:31 am

I wonder if the tree would tolerate having a bigger bite taken out of it? Trees seem to happily survive all sorts of abuse and this would be on the inside of the lean and hence under compression rather than tension.

Any arboreologists out there?

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Re: Ideas for a bridge replacemnt?

Post by gregh » Wed Aug 05, 2020 6:38 am

Lots of brilliant lateral thinking there. Thanks.
I will consider and let you know.
Greg from downunder.
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Re: Ideas for a bridge replacemnt?

Post by FWLR » Fri Aug 07, 2020 8:50 am

I like invicta280's idea about moving your track slightly to the left of the tree. It really would be a shame to cut into your beautifully built bridge Greg. And just putting a plank under the track is just the same as your first photo, when you say it is a 1m drop.... :? :(

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Re: Ideas for a bridge replacemnt?

Post by gregh » Sat Aug 08, 2020 7:20 am

I'll try and explain why it is impossible to realign the track.
The bridge supports sit on the edge of a high rock shelf. Below that rock shelf there are other tracks at Sandstone, so I cannot even build longer piers.
These pics might help illustrate.
On the right end the bridge sits on the rock but the support wall is right on the edge - no moving it out!!
right end capt.jpg
right end capt.jpg (273.5 KiB) Viewed 5398 times
The left end is on a high pier, but you can see it is impossible to move outwards.
left end capt.jpg
left end capt.jpg (223.3 KiB) Viewed 5398 times


I don't mean to be negative and thanks everyone for the ideas. All the offered ideas involve lots of work. I'm still procrastinating!
Greg from downunder.
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Re: Ideas for a bridge replacemnt?

Post by pandsrowe » Sat Aug 08, 2020 8:06 am

It probably not what you want to hear but it seems to me that it's the tree that has got to go. The angle that it's leaning at seems to indicate that its only a matter of time before it falls of it's own accord.
Phil

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Re: Ideas for a bridge replacemnt?

Post by FWLR » Sat Aug 08, 2020 8:19 am

Your'e not being negative Greg. It's a problem you have asked for ideas from members and I can now see what a quandary you have. It is looking like you may need to put a new bridge in like you have suggested yourself with an underslung one. You have the skills to make another in the same mould has the old one and maybe you can put the old one to good use somewhere else on your line..

Has for that tree..... :?

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Re: Ideas for a bridge replacemnt?

Post by Peter Butler » Sat Aug 08, 2020 10:39 am

Have you considered moving house Greg?
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Re: Ideas for a bridge replacemnt?

Post by gregh » Sat Aug 15, 2020 7:05 am

Having procrastinated long enough, I decided that since I had to remove the track and bridge in any case, I would do that and look for inspiration.
And…………
I took the easy way out and just cut further into the tree.
It was much easier to cut with the bridge out of the way! So I have got another few cm of clearance which puts off a new bridge for a few more years.
treecut 13-8a.JPG
treecut 13-8a.JPG (277.23 KiB) Viewed 5270 times


While I had the bridge out I gave it a few coats of paint.
And here it is all re-instated. Looks just the same as before!
fixed2.JPG
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Greg from downunder.
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Re: Ideas for a bridge replacemnt?

Post by FWLR » Sat Aug 15, 2020 8:16 am

Looks fabulous Greg. Will be ok now for another 20 years or more...

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Re: Ideas for a bridge replacemnt?

Post by philipy » Sat Aug 15, 2020 8:39 am

Result!
Philip

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Re: Ideas for a bridge replacemnt?

Post by Peter Butler » Sat Aug 15, 2020 10:55 am

Greg, you might have solved the problem as far as the railway and bridge are concerned, but tree surgeons are now in tears. I do hope it forgives you!
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